4 ms·

Knowing nothing about these things, are the AoA sensors accurate on the ground? I assume they depend on predictable airflow, of which there's initially none and

Knowing nothing about these things, are the AoA sensors accurate on the ground? I assume they depend on predictable airflow, of which there's initially none and Boeing have to draw the line somewhere as to when they become reliable?

lagadu 7y ago
Even if they're accurate, due to the ground effect[0] when an airplane is close to the ground it can have significantly higher AoAs without stalling because the stalling speed is so low at those heights. [0] https://en.wikipedia.org/wiki/Ground_effect_(aerodynamics) https://en.wikipedia.org/wiki/Ground_effect_(aerodynamics)
- sokoloff 7y ago
  > when an airplane is close to the ground it can have significantly higher AoAs without stalling No. A wing will stall at a consistent critical angle of attack. Ground effect works by increasing the lift available for a given angle of attack, not by allowing a larger angle of attack to develop. Parent comment was asking about the need for minimum airspeed to set the AoA sensors into a reliable state such that they could be checked at the gate, though, which is unrelated to ground effect.
  - FabHK 7y ago
    Not necessarily at the gate, but possibly during taxi and definitely during the takeoff roll (ideally before V_1). Granted, btw, that AoA readings on the ground may be unreliable - the question remains whether AoA disagree could be detected reliably before takeoff.
    - sokoloff 7y ago
      If the issue you're worried about is only MCAS related, it seems like the takeoff roll might be a poor place to want to detect it. An abort just prior to V1 is a no-joke exercise from a safety standpoint. MCAS operation is suppressed with any flaps deployed, so taking an airplane with a failed AoA sensor into the air and troubleshooting/returning to the departure airport if needed or simply flying with an INOP MCAS may very well be safer than a high-speed abort.
      - FabHK 7y ago
        Good point. Instead of aborting take-off, either do a circuit back WITHOUT retracting flaps, or set Stab Trim switch to CUTOUT and fly to destination trimming manually with the trim wheel? Wonder what pilots are briefed to do currently (well, if MAXes were flying).

lagadu 7y ago

Even if they're accurate, due to the ground effect[0] when an airplane is close to the ground it can have significantly higher AoAs without stalling because the stalling speed is so low at those heights. [0] https://en.wikipedia.org/wiki/Ground_effect_(aerodynamics) https://en.wikipedia.org/wiki/Ground_effect_(aerodynamics)

sokoloff 7y ago
> when an airplane is close to the ground it can have significantly higher AoAs without stalling No. A wing will stall at a consistent critical angle of attack. Ground effect works by increasing the lift available for a given angle of attack, not by allowing a larger angle of attack to develop. Parent comment was asking about the need for minimum airspeed to set the AoA sensors into a reliable state such that they could be checked at the gate, though, which is unrelated to ground effect.
- FabHK 7y ago
  Not necessarily at the gate, but possibly during taxi and definitely during the takeoff roll (ideally before V_1). Granted, btw, that AoA readings on the ground may be unreliable - the question remains whether AoA disagree could be detected reliably before takeoff.
  - sokoloff 7y ago
    If the issue you're worried about is only MCAS related, it seems like the takeoff roll might be a poor place to want to detect it. An abort just prior to V1 is a no-joke exercise from a safety standpoint. MCAS operation is suppressed with any flaps deployed, so taking an airplane with a failed AoA sensor into the air and troubleshooting/returning to the departure airport if needed or simply flying with an INOP MCAS may very well be safer than a high-speed abort.
    - FabHK 7y ago
      Good point. Instead of aborting take-off, either do a circuit back WITHOUT retracting flaps, or set Stab Trim switch to CUTOUT and fly to destination trimming manually with the trim wheel? Wonder what pilots are briefed to do currently (well, if MAXes were flying).